ntroduction to acid fracturing technology

In the process of stimulation of reservoir production, two major types of technologies, matrix acidification and fracturing and acidification, have been formed. Matrix acidification, also known as conventional acidification, refers to the acidification technique of injecting acid into a formation, relieving damage near the wellbore, and restoring reservoir productivity, under conditions where the pressure at the bottom of the well is less than the fracture pressure of the reservoir. Fracturing and acidizing, also referred to as acid pressure, means that when the pressure at the bottom of the well is higher than the rock burst pressure of the formation, the acid is used as the fracturing fluid and no formation of any proppant is injected into the formation. The formation will crack. The acid flows in the crack and reacts with the wall of the crack, dissolving the crack surface into a bumpy form. The surface characteristics of the acid fractures are strongly dependent on the acidification environment, including the type and strength of the fracturing fluid, the acid erosion rate, the acid fluid filtration rate, and the rock type. When the pump is stopped and the pressure is released, the crack wall can not be completely closed, so that cracks with certain conductivity and permeability are formed in the formation. Acid fracturing

Two important factors that control the effect of acid pressure stimulation of carbonate rock are the effective length of fracture and the conductivity of fracture. The former is affected by acid fluid loss, acid rock reaction rate, acid flow rate and acid type. The latter is affected by fracture closure and erosion pattern of acid rock reaction. A series of acid fracturing techniques have been developed to increase the production of oil and gas for effective diversion and longer acid fractures.

1 acid fracturing technology status quo

In accordance with the process of technology, fracturing technology can be divided into pickling, acidification and acid pressure of three. Acid pressure in order to achieve fluid loss control, slow the acid rock reaction rate, the formation of a long acid etching cracks for the division of standards, divided into ordinary acid pressure, acid pressure balance, acid pressure and the alternating pre-injection acid pressure

2 acidification fracturing technology

 Acidification fracturing process is divided into pre-fluid, thickened acid pressure, multi-level alternating injection and closed acidification combination of technology.

2.1.1 pre-fluid

The pre-liquid acid pressure is also called sticky fingering, that is, using high-viscosity liquid to open the formation, forming a certain size cracks at the same time reduce the temperature of the rock surface, delaying the effective distance between the acid and the rock to make the crack more long. In the pre-liquid acid pressure, the acid will displace the viscous "fingering" effect when displacing high viscous fracturing fluid, and the acid solution will produce uneven etching on the fractured wall to form the groove so that the fractures have higher flow conductivity day . Viscosity ratio of the preflush and acid determines the smoothness of the viscous fingerprinting. The viscosity ratio is too small, and the flow rate of the acid will be greater than that of the preflush and can not be "fingered in." Pre-liquid acid pressure process to pay special attention to the formation of filter cake or not, once the crack wall to form a filter cake, the filter cake can be a very small amount of acid quickly penetrate and produce acid corrosion holes, the acid loss at this time And with no forerun is the same

2.1.2 thickened acid pressure

There are generally two kinds of thickening acids: non-crosslinking acid thickener and crosslinking gel acid. The thickened acid is a highly viscous acid liquid system formed by adding a water-soluble high-boiling point acid to an acid, usually consisting of an acid, a thickener, and other additives. Among them, hydrochloric acid is the most commonly used acid for the preparation of thickening acids. During acidification, depending on the nature of the formation to be treated, the type of acid used and the acid concentration can be determined. Thickened acid is generally suitable for the acid pressure of the medium permeability reservoir, acid corrosion seam length of 20-50m asked, to be used with caution in the hypertonic and low permeability thickened acid. In addition, due to the viscosity, the use of thickening acid to consider back to the problem. In the acid pressure, the thickening acid can improve acid penetration ability, increase fracture conductivity, reduce acidification secondary pollution.

2.1.3 closed acid fracturing

A small amount of acid is pumped into the submerged fractures to create as many open fractures as possible with the wellbore to create fractures around the wellbore. The acid poured into the closed cracks in the process should pay attention to the acid can not exceed the required standard so that the well and bottom hole air layer can be effectively combined